This application claims the priority of German application 197 46 698.2, filed in Germany on Oct. 22, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a reactor which is constructed essentially cylindrically symmetrically about an essentially vertically running axis and has a shell and within the shell at least one annular bulk packing which is filled with free-flowing material and is bounded by an inner and an outer grating and, on its lower side, by a plate supported from the bottom on the shell.
The invention further relates to the use of the reactor according to the invention for pressure swing adsorption processes.
For reactors of the generic type, as are disclosed, for example, by EP-B 0 402 783--there is a further field of application. They can be used for the most varied types of reactions between a gas and an active material which is present in free-flowing form. The active material can be, for example, an adsorbent or a catalyst. The reactor can comprise a plurality of types of active material and can consist of more than one bed or one bulk packing. A bulk packing in this case is concentrically enclosed by an adjacent bulk packing.
During the reaction phase, a reaction gas is led roughly radially to the axis of symmetry of the reactor through the bed filled with active free-flowing material by passing, it, for example, to the space between shell and outer basket or outer grating and taking it off again from the space within the inner basket or inner grating. In the case of an adsorption reaction, the reactivity of the active material (adsorbent) decreases with increasing reaction time. Therefore, the adsorbent must be regenerated at regular time intervals.
This regeneration of the adsorbent can be carried principle in two different ways. Thus, firstly, during the regeneration phase, a regeneration gas, which in comparison with the gas to be purified has a different chemical composition and/or a different thermodynamic state, can be passed through the bulk packing of active material. Generally, the regeneration gas passed through the bulk packing of active material has a higher temperature than the gas mixture passed through the bulk packing during the adsorption phase; this is then termed a T(emperature) S(wing) A(dsorption) ("TSA") process. However, the laden adsorbent can also be regenerated by a pressure decrease within the bulk packing; this is then termed a P(ressure) S(wing) A(dsorption) ("PSA") process. Obviously, those skilled in the art know a multiplicity of mixed forms of TSA and PSA processes.
The reaction taking place in the reactor can consist, e.g. of a separation of gas mixtures by adsorption or of a removal of unwanted constituents by adsorption from a gas to be purified. A practical example of the latter is the separation of water and/or carbon dioxide from air which is fed to a low-temperature air fractionation plant. The free-flowing material which is introduced into the packed bed acts as adsorbent in this case and can, for example, consist of a molecular sieve, zeolite and/or alumina gel.
During the reaction phase or adsorption phase, the air to be purified is led through the bed or the bulk packing and water and/or carbon dioxide is given off in this operation to the active material, that is to say is adsorbed to this. During the regeneration phase, the substances removed from the air are desorbed again, by passing a regeneration gas, for example nitrogen, through the bed containing the adsorbent. In this operation, generally, as described above, temperatures and/or pressures other than those during the adsorption phase prevail.
A reactor of the type mentioned at the outset can, in addition, be used for catalytic reactions, for example for the removal of No.sub.x from the exhaust gas of combustion plants. The free-flowing material in this case consists, for example, of metal-doped molecular sieve particles.
A central problem in the construction of a reactor of this type is that what are termed bypass flows must be avoided in the top area of the reactor. For this purpose, in the top area of the reactor, a dead space containing bulk packing material through which flow does not pass, or only slightly, is provided, which dead space compensates for the compression of bulk packing by 2 to 5% which is unavoidable after the filling and start-up, and the associated fall of the bulk packing.
Reactors of the type mentioned at the outset are also used at high throughput with pressure-swing regenerated adsorption plants, such as in what are termed "heatless dryers" or vacuum-regenerated PSA processes for O.sub.2, production from air or for CO, separation from blast furnace gases. In these PSA processes, the pressure drops in the regeneration phase must be kept as low as possible. In particular during an adsorption operation from outside to inside--this means that the feed gas flows, through the bulk packing radially from outside to inside--and regeneration by pressure decrease and accompanying flushing from inside to outside, in each case at the point of greatest gas input, the area of passage is also greatest and thus the overall pressure drop is lowest. This fact effectively reinforces the desorption and thus improves the working loading.
Since, in the case of pressure-swing regenerated adsorbers, only low temperature differences arise due to heats of adsorption and desorption, the gratings for holding the bulk packing can be constructed, for example, as perforated sheets without special measures for compensating for reaction to temperature changes; this considerably reduces the reactor manufacturing costs.
However, the central problem when radial flow reactors are used is, in the case of PSA processes, that the losses from the upper dead space zone due to the then frequent pressure changes--the switch-over between adsorption and desorption phases is carried out at intervals between 0.5 and 10 minutes--become marked and, as a result, when the known reactor structures, which are used, in particular, in the case of TSA processes, are simply applied to PSA processes, attractive design is no longer possible.
An object therefore underlying the invention is to develop further the reactor of the type mentioned at the outset to the extent that, with simultaneous avoidance of a bypass in the top area of the reactor, as little as possible bulk packing through which flow does not pass in adsorption or reaction remains as dead space, so that the losses of product components can be reduced during the pressure decrease.
This object is achieved according to preferred embodiments of the present invention by virtue of the fact that means are provided for widening the bulk packing in the top area of the reactor.
These means for widening the bulk packing in the top area of the reactor can be constructed in accordance with an embodiment of the reactor according to the invention, e.g. in the following way: in the upper area of the reactor, the outer grating is joined to the reactor shell in a gas-permeable manner and the inner grating or gratings widen conically outwards.
These structural measures achieve a widening of the bulk packing in the top area of the reactor which, firstly, decreases the fall in bulk packing due to compression and decreases the associated bypass risk, due to the base area expanded above. Secondly, a considerable simplification in construction in comparison with known reactor structures having dome-shaped outer and inner gratings is achieved. In addition, in this manner, a uniform flow in the top area through the conically widened inner grating or gratings is achieved, as a result of which the resistance to flow is decreased precisely in the top area of the reactor through which flow previously passed poorly, due to the substantial increase in the inner area of passage.
According to advantageous embodiments of the reactor according to the invention, the inner grating or gratings widen outwards at an angle of from 1.degree. to 45.degree. to the essentially vertically running axis. The outer or outermost grating is--developing further advantageously the reactor according to the invention--joined to the reactor shell at an angle of from 0.degree. to 45.degree. to the horizontal.
In a further development of the reactor according to the invention, it is proposed that the inner grating or gratings preferably widen outwards at the height at which the outer grating is joined to the reactor shell in a gas-permeable manner.
The distance between the joint between the outer grating and the reactor shell and the upper reactor rim is preferably between 80 and 130% of the radial packing thickness of the bulk packing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.